How do I configure GDB correctly, to stop error "You can't do that without a process to debug"

Question

Ubuntu 16.04.4 LTS, GNU gdb (Ubuntu 7.11.1-0ubuntu1~16.5) 7.11.1

I am trying to call a function in a compiled C program and get the following:

"(gdb) call getVarName(someParam)
 You can't do that without a process to debug."

There are no other codes or messages.

I can run the program from the shell prompt jef@ubuntu$ ./program. I can run the program within gdb after designating the file. Permissions are 777 (just to cover all bases).

Based on research, I set the SHELL with "export SHELL=/bin/bash" and set kernal.yama.ptrace_scope = 0 in /etc/sysctl.d/10-ptrace.conf

I still get the same behavior.

Can you show the command line you invoked gdb with, and all relevant gdb commands you ran prior to the `call` ? — Mark Plotnick, Apr 06 '18 at 15:48
(gdb)file program (gdb) break main Breakpoint 1 at 0x400994 (gdb) — user1660886, Apr 06 '18 at 15:50
i can also step through with "step" and "next" after setting a break point and "run". "call" is what is giving me fits. — user1660886, Apr 06 '18 at 16:07
The root cause here is that the **OP forgot to `run` the program first**. For core dump see [c - gdb evaluate function in process core - Stack Overflow](https://stackoverflow.com/questions/8892597/gdb-evaluate-function-in-process-core) instead. (no solution) — user202729, Mar 10 '22 at 00:59

score 3 · Accepted Answer · answered Apr 06 '18 at 16:15

3

I still get the same behavior.

Naturally.

The error you are getting means: you can't do this, unless you are debugging a live process.

This will work:

(gdb) break main
(gdb) run
... GDB is now stopped, *and* you have a live process.
... you *can* call getVarName(...) now
(gdb) call getVarName(...)

(gdb) continue  # causes the process to run to end and exit
[Inferior 1 (process 195969) exited normally]

(gdb)  # Now you no longer have a live process, so you *again* can't
       # call functions in it.

answered Apr 06 '18 at 16:15

Employed Russian

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Yes, that worked. Thanks!! It makes sense now that I am out of the weeds and tunnel vision. I thought I could just call a function, but upon reflection what is the point of a break if that was possible... Now I get why there are a lot of forum posts with this error around core files. – user1660886 Apr 06 '18 at 16:43
I have a question related to this one. I'm debugging `core dump` with gdb and tried to do `print errno` to get the error code. I got `You can't do that without a process to debug`. Can you suggest some idea about why isn't it possible to print `errno`? – St.Antario Jan 19 '19 at 07:09
1

@St.Antario That's a completely separate question. Please ask one, it doesn't cost you anything. – Employed Russian Jan 19 '19 at 07:23

How do I configure GDB correctly, to stop error "You can't do that without a process to debug"

1 Answers1